Diffusion pump assembly



Mrh 2, 1965 cHzKARA HAYAsl-u 3,171,584

DIFFUSION PUMP ASSEMBLY Filad may 14, 1952 ATTORNEY United States PatentO Y 3,171,584 DWFUSEN PUMP ASSEMBLY Chikara Hayashi, Koholm,Yokohama-shi, Slapen, assigner to Nihon Shinku 'Giiutsu KabushikiKeisha, Yokohamashi, Japan, a hrm Filed May 14, 1952, Ser. No. 194,553Claims priority, application Japan, May 16, 1961, 3io/16,836; May 2,1962, 37/17,489 2 Claims. (CI. 23h-4S) This invention relates to animproved diffusion pump provided with a novel means for protecting avacuum chamber from backstreaming vapor. Backstreaming constitutes aback migration of vapor molecules into the vacuum chamber and isinherent in a diffusion pumping process.

Attainable orders of vacuum or absolute pressure to be establishedwithin the vacuum chamber is the sum of the partial pressure of thebackstreaming vapor from the pump and the partial pressures of gasesevolved from the walls of the vacuum chamber and members mountedtherein, and/ or gases introduced therein from the exterior. The partialpressures of gases except that of the backstreaming vapor, if beingconstituted of a single component, equals to the ratio of the amount ofthe gases, measured in the unit or" absolute pressure X volume/hour, tothe effective pumping speed of the pump. In case the gases consist ofseveral components, the gas pressure is the sum of their respectiveratios as above-mentioned.

As seen from the above description, the rising of the orders of vacuum,in other words, the lowering of the absolute pressure Within the vacuumchamber Wholly depends on a lowering of the two partial pressures. Asultra-high vacuum engineering has developed, the emission of gases fromthe elements or members of the vacuum chamber and/or permeation of gasesthrough the walls of the vacuum chamber have been sufficiently reduced,and manufacturing of pumps having large pumping speed has beenfacilitated. Therefore, attainment of higher orders of vacuum mainlydepends on a lowering of the amount of backstreaming vapors developedfrom die Working fluid.

Heretofore, to reduce the partial pressure of the backstreaming vapors acold trap means located between the vacuum chamber and the diffusionpump to catch or hold the backstreaming vapors has been used. The coldtrap, however, has disadvantages such that' resistances to the pumpingaction are increased, consequently the effective pumping speed of thepump will be decreased, and the construction of the pumping assemblywill be complicated, and such that when or after the operation of pumpis stopped, the condensed volatile component of working uid within thecold trap Will often be reevaporated. Another means is a type of anadsorption trap. The disadvantage of the adsorption trap is that whenthe adsorption agents are saturated within the vapors of working fluid,the effect of the trap will be decreased. Therefore, some means isurgently required to prevent the baclcstreaming of vapors of the workingfluid into the system being pumped by the diffusion pump.

An object of the present invention is to provide a novel and usefuldiffusion pump provided with means for positively preventing thebackstreaming of vapors.

The principle on which the invention is based is Raoults law. For betterunderstanding of the invention, Raoults law is concisely referred to.The Working fluid to be used in the diffusion pump is evaporated andcondensed in the system of the pump, and generally consists of manycomponents. For the sake of brevity, however, we may assume that theyare divided into two groups, one of which has a low vapor pressure andthe other a high vapor pressure at their working temperatures. Theformer usually has a large molecular weight in comparison with thelatter, and constitutes the main part of the Working fluid. Hereinafter,the former is referred to as a main component and the latter as a lightdistillate, because the latter is of light and quickly evaporated andcondensed in comparison with the former by heating and cooling. Theexistence of the light distillate of high vapor pressure is resultedfrom the insuilcient purification of material and/or caused by thedecomposition of the working fluid due to heat supplied to the pump.

Let Pa and Pb be the vapor pressure of the main component and the lightdistillate respectively, and MazMb be a molecular ratio of the twocomponents, then the following relations generally hold between themWhen the Working uid is condensed after evaporation, some part of thecondensed product is reevaporated and subiccted to the cyclic process ofcondensation and evaporation along the pipe line connection thediffusion pump and the vacuum chamber, and finally a part of themarrives at the interior of the vacuum chamber.

According to Raoults law, the ratio of the two components after a firstevaporation is PaMazPbMb, and after a second evaporation Pa2Ma:Ph'2Mband after a nth evapration PanMarPbnMb. Now let Pn be the total vaporpressure over the nth condensed liquid phase, it can be expressed asThen, let lr be the partial pressure of the backstreaming vapor in thevacuum chamber, and P0 be the vapor pressure of the Working fluidcondensed with the diffusion pump, then the approximate values of Pr andPD may be derived from Formula l by putting n=n(n l) and n=0,

(2) Pr: nla at f1 1 (3) PoPaMa-kPbMb at 11:0

As mentioned above, MaMb and Ma-l-Mb=1, therefore P0 is sufficientlysmaller than Pr.

The inventor has perceived this point and invented a novel means able toreduce Pl. to P0 which corresponds to the partial pressure appearing inthe vacuum chamber in case there is no backstreaming, in other words,means able to reduce the amount of the backstreaming to zero.

It is to be noted that the above description is made in reference to alight group composed of a single component, but the same principle canbe applied to a group composed of several components.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawings wherein:

The single figure is a diagrammatic sectional view of the invention. I

Referring t'o the single figure there is shown one ernbodiment of theinvention where reference numeral 10 indicates a backstrearning checkdevice which serves to prevent the backstrearning vapor of theworkingfluid to tend toward a vacuum chamber to be evacuated and whichis able to serve as a high vacuum side diffusion pumping device. Thebackstreaming check device 10 comprises an upright cylindrical casing l2provided with a head cover il and a boiler 13 mounted Within and at thebottom of the casing l2. The boiler 13 comprises a heater 14, and outercylinder 15, a cylindrical ejector coaxially disposed with the outercylinder 15. The interior of the outer cylinder is divided by both ahorizontal partition 17 and the ejector lo into three chambers such asan upper chamber 1?, a lower chamber 19 and an evaporation chamber 2t),and the lower chamber 19 is communicated with the evaporation chamber 20through opening 21 formed in the lower portion of ejector 16. Around theupper portion 22 of the outer cylinder above the partition 17 isproviding a cooling means 23. The cylindrical ejector 16 is formed tohave at its upper portion a flare adapted for an effective ejection ofthe evaporated working fluid, and at its lower la diametrically enlargedsection to increase the amount of the vapor of the working fluid. Byproviding an inclined plate 24 Within and at the upper portion of thecasing 12, there is formed a jet chamber 25. A cooling means 26 ismounted to cover the most part of the casing 12 under the inclined plate25. An inlet port 27 is formed at the lower portion of the casing toconnect the backstreaming check device 10 to a vacuum chamber C shown inimaginary line. The casing 12 is communicated with the lower chamber 19of the boiler 13 by a return tube 28 which extends outward from thebottom of the casing 12 and terminates at a portion adjacent the bottomof the boiler 13.

The backstreaming check device 10 is connected to a 'diffusion pump 4t),which is described hereinafter, by a connecting pipe 30 horizontallyextending from a portion of the casing 12 underneath the upper edge ofthe inclined plate 24, a distillation pipe 33 extending upward from thelower portion of the outer cylinder 15 above the liquid level of theworking fluid passing a heater 31, turning downward and passing througha cooling means 32, and other connecting pipe 34 extending from aportion of the outer cylinder 15 under the liquid level of the workingfluid.

A difusion pump generally indicated by reference numeral 4) is aconventional diitusion pump of the distillation type comprising a casing41 yand a boiler unit 42 enclosed therein. The casing 41 provided with acooling means 43 and an outlet port 44 to be connected with a back pump(not shown). The boiler unit 42 includes an outer cylinder 45, an innercylinder 46 coaxially disposed With the outer cylinder 45 and a heatingmeans 47. The working uid ofV liquid phase in the diffusion pump issubstantially separated by the walls of the outer and inner cylindersand 46, but partially communicated With each other through openings 55and 56 formed in their respective walls. The working fluid stored at thebottom of the diiusion pump 4i) is heated to its evaporation temperatureby a heating means 47. The vapor produced from the working fluid withinan outer chamber 50 is downwardly ejected through `an annular opening 51into a pumping chamber 57 deiined by the casing 41 and the boiler 42.The other vapor produced within the inner cylinder 46 is similarlyejected through openings 53 vand 54. These ejected vapor Vstreams effectto catch the gas molecules coming from the vacuum chamber C, through theconnecting pipe 3), into the diitusion pumpV 40, and the most part ofthe ejected'vapors is condensed on the inside surface of the casing 41being cooled by the cooling means 43, zand then flows down along theWall of the casing 41 and inally is collected at the bottom of thepumping chamber 57, While a part of the ejected vapors is drawn, by thepumping action of the back pump, through the outlet port 44 into theside of the backing pump. The condensation of Working fluid accumulatedat the bottom of the pumping chamber S7 should arrive `at the innerchamber 52 after passing through a roundabout passage which is formedonly by provision of the openings 55 and 56. The working iluid issubjected to fractional distillation while passing the passage, and as aresult the working iluid arrived at the inner cylinder 52 has a 10Wvapor pressure at the working temperature and a large molecular Weight.This low vapor pressure working fluid is supplied into the lower chamber19 of the boiler 13 by means of the connecting pi'pe 34.

Y In operation of the diffusion pump assembly, the inlet port 27 of thebackstrearning check device 10 is connected to the vacuum chamber C, theoutlet port 44 of the diiusion pump 44B is connected to the back pump(not shown) and then the back pump is operated. When the interiors ofthe vacuum chamber C, the backstreaming check device 1t? and the diusionpump 40 are evacuated by the back pump to a vacuum adapted for startingof the diffusion pump 40, cooling mediums of appropriate temperaturesare supplied from ia source or their respective sources, such asrefrigerating machines or cooling pumps, to the-cooling means 26, 23, 32and 43 respectively; the heating means 14 and 47 are energized by anelectric source(s) to evaporate the working iluids within thebackstreaming check device 10 and the ditusion pump 49 and at the samethe heater 31 is energized to accelerate Flowing of the vapor in thedistillation pipe 33; and thus the backstrearning check device 10 andthe diffusion pump 46 are set in operation. By pumping actions of thediffusion pump 4i) and the back pump, the gas molecules within thevacuum chamber are drawn into the ejection chamber 25 and directed tothe connection pipe 36 by the vapor upwardly ejected from the flaredopening of the ejection cylinder 18, and' introduced into the diffusionpump 41B by way of the connection pipe 36, and subjected to the pumpingaction of the ditusion pump and finally drawn through the outlet port 44into the system of :the back pump.

It is to be noted that the vapor ejected from the flared opening of theejection cylinder 16 Consists of a vapor of low vapor pressure and largemolecular Weight, because the working huid from which the ejected vaporis produced, has already been subjected to a fractional distillation byheat before arriving at the ejection chamber 16. The main part of theejected vapor shall be condensed on the inclined plate 24 which, ifnecessary, should be kept at an appropriate temperature to effectfurther removal of the trace of lighter component in the condensate. Thelighter component in the condensate on the plate 24 is subjected toreevaporation and migrates through the tube 30 .to the diiusion pump 40.AS a whole the plate 24 expresses a simple means of purication ot thecondensate. Therefore its effect can be amplied or intensified by usingan additional distillation tower or equivalents. Thus the puriedcondensate which is composed of enriched low vapor pressure componentiiows down along the casing 12, suiciently cooled by a cooling means 26and arrives at the bottom of the ejection chamber 25 without beingreevaporized, and arrives at the lower outer chamber 19 of the boilerunit 13 by means of the return pipe 28 and ows through the opening 21into the original chamber or evaporation chamber 20.

As is apparent from the above description, the most part of the ejectedvapor consists of the vapor having a low vapor pressure at the workingtemperature and a large molecular weight, but yet it involves a smallportion of the vapor having a high vapor pressure at the Workingtemperature, which vapor is produced from the light distillate.

The upper portion 22 of the outer cylinder 15 including the coolingmeans 23 serves as a heat insulator for the casing 12 and to condensethe vapors impinged thereon.

Referring now to evaporation, condensation and evaporation of theworking iluid to be circulated between the backstreaming device 10 andthe diffusion pump 40, when the working fluid (liquid) within the lower,outer chamber 19 is heated, the component involved therein and having ahigh vapor pressure is first evaporated, and the working uid becomesrich in the component having a low vapor pressure.

The working tluid rich in the main component flows through the opening21 into the evaporation chamber Z0 of the cylindrical ejector 16, and isconverted by the eating means l into the vapor to be ejected from thelared opening of the cylindrical ejector 16. The ejected vapor may actas a iine side vapor jet of a diffusion pump. The ejector' i6 maytherefore be equipped with .a multistage jet assembly which is moreelective as a pumping means. Thus the baclrstrearning check device lilitself is a diit'usion pump. rEhe ejected vapor returns to the originalchamber after being subjected to the fractional distillation, as abovementioned. The vapor produced by the fractional distillation of ltheworking liuid, and having a high vapor pressure rises in the lightdistillate pipe 33 and is heated by the heater 3l to accelerate themigra ion of the lighter component to the portion 32 where they arecondensed by the cooling means 3;, and flo-w into the diffusion pump 4@and is collected at the bottom of the pumping chamber 57. As describedhereinbefore, the vapor oi the light distillate in the boiler fr? andthe gas molecules evolved in 19 introduced through the connecting pipeSi@ linto the diffusion pump llt?, and the vapor produced from the mainworking iuid and ejected from the openings 51, 53 and d also arecondensed and collected at the bottom of the pumping chamber S7.Therefore, the Working fluid in the pumping chamber 57 contains arelatively large amount of the light distillate, but this lightdistillate may be evaporated during passing its roundabout passage, andthe working fluid becomes rich in the main component when it arrived atthe inner chamber 52. The resultant Working iiuid is returned to thelower, outer chamber i@ by means of the connecting pipe 34 and subjectedto another fractional distillation as described hereinbefore. T ius,both the baclrstrearning checl: device 1t) and the diiusion pump eiiectthe fractional distillations of the common working tiuid and act thepumping actions in conjunction with each other. Now that the inside Wallof the lower part of the casing l2 is always kept cool and is coveredwith a fresh condensate flowing down from the inclined plate 24 to thereturn tube 28. The fresh condensate is mostly composed of the low vaporpressure iiuid which will not have any appreciable migration velocity tothe chamber C. A lighter component backstreaming from the vapor jet from16 lis eiectively caught by the cold low vapor pressure condensate andits migration to the chamber C is kept minimum as proved by the Raon tstheorem.

A practical example is listed to show the merit of the invention.

Vacuum in torr: Pumping speed in liter second The above data wereobtained by using a diffusion pump assembly of the type shown in FIG. l,and having the following particulars:

Backstreaming check Cooling means Working tluid Rate of backstream atthe pump inlet 'Less than 0.601 cc./hr.

As is apparent from the practical data above-mentioned, the diffusionpump in accordance with the present invention, such advantages that, itis to obtain higher vacua than those obtainable by previous diffusionpumps and there are no diiiiculties such as a backstreaming of vaporwhich diiculties are inherent in diusion pumps.

It is to be noted that a coolant to be used in the cooling means may bewater trom a water source or cooling medium directly supplied from arefrigerator or brine cooled by the cooling medium from a refrigerator,A thermoelectric cooling element can also be used as a cooling means.

The back pump referred in the description may be a rotary pump or adevice employing a molecular sieve or any other auxiliary pump.

Si ce certain changes may be made in the above apparatus and processWithout departing from the scope 0f the invention herein involved, it isintended that all matter contained in the above description, or shown inthe accompanying drawings, shall be interpreted as illustrative and notin a limiting sense.

What i claim is:

l. A diiusion pump assembly comprising a first diffusion pump incommunication with a chamber to be evacuated and a second ditusion pumpadapted to be placed in communication with a backing pump, said pumpsbeing multi-cha mbered and in series communication with each other, eachpump having means for vaporizing the organic working liquid employedtherein to catch backstreaming gases, high vapor pressure component insaid liquid being fractionated out in said rst pump in one chamberthereof, a second chamber in said iirst pump for vaporizing the lowpressure component of said liquid into the main gas stream, means forconducting said thus separated high vapor pressure component to saidsecond pump into the main gas stream Without contaminating the mainstream in the first pump, a chamber in said second pump for firstfractionating out the high vapor pressure component from the liquidtherein into the main gas stream, a second chamber in said second pumpfor then vaporizing the low vapor pressure component into the main gasstream, means for conducting liquid from said second chamber of saidsecond pump to said iirst mentioned chamber of said irst pump withoutcontaminating the main gas stream.

2. A device in accordance with claim l wherein cooling means areemployed at various connecting points in the system to recondense vaporin these regions and prevent the backstreaming of gases.

LAURENCE V. EFNER, Primary Examiner. WARREN E. COLEMAN, Examiner.

1. A DIFFUSION PUMP ASSEMBLY COMPRISING A FIRST DIFFUSION PUMP INCOMMUNICATION WITH A CHAMBER TO BE EVACUATED AND A SECOND DIFFUSION PUMPADAPTED TO BE PLACED IN COMMUNICATION WITHA BACKING PUMP, SAID PUMPSBEING MULTI-CHAMBERED AND IN SERIES COMMUNICATION WITH EACH OTHER, EACHPUMP HAVING MEANS FOR VAPORIZING THE ORGANIC WORKING LIQUID EMPLOYEDTHEREIN TO CATCH BACKSTREAMING GASES, HIGH VAPOR PRESSURE COMPONENT INSID LIQUID BEING FRACTIONATED OUT IN SAID FIRST PUMP IN ONE CHAMBERTHEREOF, A SECOND CHAMBER IN SAID FIRST PUMP FOR VAPORIZING THE LOWPRESSURE COMPONENT OF SAID LIQUID INTO THE MAIN GAS STREAM, MEANS FORCONDUCTING SAID THUS SEPARATED HIGH VAPOR PRESSURE COMPONENT TO SAIDSECOND PUMP INTO THE MAIN GAS STREAM WITHOUT CONTAMINATING THE MAINSTREAM IN THE FIRST PUMP, A CHAMBER IN SAID SECOND PUMP FOR FIRSTFRACTIONATING OUT THE HIGH VAPOR PRESSURE COMPONENT FROM THE LIQUIDTHEREIN INTO THE MAIN GAS STREAM, A SECOND CHAMBER IN SAID SECOND PUMPFOR THEN VAPORIZING THE LOW VAPOR PRESSURE COMPONENT INTO THE MAIN GASSTREAM, MEANS FOR CONDUCTING LIQUID FROM SAID SECOND CHAMBER OF SAIDSECOND PUMP TO SAID FIRST MENTIONED CHAMBER OF SAID FIRST PUMP WITHOUTCONTAMINATING THE MAIN GAS STREAM.